The present invention relates to a data collection method and an MRI (Magnetic Resonance Imaging) system, more specifically, to a data collection method and an MRI system which can reduce artifacts caused by sudden change of a phase encode amount.
FIGS. 11 and 12 are explanatory views showing the data collection method disclosed in Japanese Unexamined Patent Publication No. Hei 10-5191 and U.S. Pat. No. 3,051,374.
First, as shown in FIG. 11, a k space K is divided into seven regions in total of a zeroth region C positioned in the center of the phase encode axis direction, two first regions L adjacent to the outside of the zeroth region C, two second regions M adjacent to the outside of the first regions L, and two third regions H adjacent to the outside of the second regions M.
The phase encode amount of the zeroth region C is, e.g., −7 to +8. The phase encode amounts of the first regions L are, e.g., −47 to −6 and +9 to 48. The phase encode amounts of the second regions M are, e.g., −87 to −46 and +49 to 88. The phase encode amounts of the third regions H are, e.g., −127 to −88 and +89 to 128. The phase encode amount is a value conceptionally representing the polarity and magnitude order.
Data collection is performed in the order as shown in FIG. 12.
In other words, data H1 of the third region H, data M1 of the second region M, data L1 of the first region L, and data C1 of the zeroth region C are collected.
With j=2, data Hj of the third region H and data C3j-4 of the zeroth region C are collected.
Data Mj of the second region H and data C3j-3 of the zeroth region C are collected.
Data Lj of the first region H and data C3j-2 of the zeroth region C are collected.
While changing j to 3, 4, . . . , data collection and image reconstruction are repeated.
After collecting the zeroth region C data, the image reconstruction is performed using the latest zeroth region C data, the past latest third region H data, second region M data, and first region L data. In the reconstructing method, as shown in FIG. 13, an image f (H1, M1, L1, C1) is first reconstructed. Then, with j=2, an image f (Hj, Mj-1, Lj-1, C3j-4) is reconstructed, an image f (Hj, Mj, Lj-1, C3j-3) is reconstructed, and an image f (Hj, Mj, Lj, C3j-2) is reconstructed. While changing j to 3, 4, . . . , the image reconstruction is repeated.
Japanese Unexamined Patent Publication No. Hei 10-5191 and U.S. Pat. No. 3,051,374 disclose other various image reconstructing methods.
In the prior art data collection method shown in FIGS. 11 and 12, the zeroth region C data is sometimes collected immediately after collecting the third region H data. At this time, the phase encode amount of the pulse sequence is suddenly changed, e.g., from +89 to +8. In addition, the zeroth region C data is sometimes collected immediately after collecting the second region M data. At this time, the phase encode amount of the pulse sequence is suddenly changed, e.g., from +49 to +8.
However, when the phase encode amount is changed suddenly immediately before collecting the zeroth region C data governing an image, artifacts appear on the image by the influence of remaining magnetization or overcurrent.